Pretty simple to test....
1. Got a test light? Great. Connect the ground lead to the NEGATIVE battery TERMINAL, not the post, and probe the POSITIVE battery TERMINAL, not the post. Lights right up proceed to #2. Doesn't light up: Fix connection problem.
2. Leave the ground lead connected to the NEGATIVE battery terminal and probe the LARGE POST on the starter solenoid facing the battery. Lights right up proceed to #3. Doesn't light up: Fix connection problem or bad cable.
3. Disconnect STARTER CABLE from starter solenoid. Attach probe to LARGE POST on the starter solenoid facing the rear. Have someone turn the ignition key to "START". Solenoid clacks and test light comes on proceed to #4. Doesn't light up: Proceed to #3a.
3a. Attach probe to small nut at the bottom of the starter solenoid "S" post (front-most small post) and have someone turn the ignition key to "START". Lights up but no "Clack" from the solenoid, check solenoid ground. If okay, replace solenoid. Doesn't light, check continuity of circuit from solenoid back to ignition switch "S" post, including Neutral Safety Switch, if automatic transmission or jumper wire in NSS harness if manual transmission. Assuming everything is fine here, use test light to probe ignition switch "S" post when key turned to "START". If test light comes on but solenoid doesn't "Clack" you still have an open circuit between the ignition switch and the solenoid. If the test light DOESN'T come on, probe the ignition switch "B" post. If no juice there, check continuity from there back to the LARGE LUG at the front of the solenoid.
4. Reconnect starter cable to starter solenoid. Turn key to "START" position. If starter doesn't crank, probe the large lug on the starter motor when key turned to the "START" position. If light DOES NOT come on, replace starter cable. If light comes on, check ground between starter housing and bellhousing separator plate/bellhousing. If ground is okay, replace the starter.
Phew...
